PERMEABILITY DEVELOPMENT IN VESICULATING MAGMAS - IMPLICATIONS FOR FRAGMENTATION

Fragmentation, or the ''coming apart'' of magma during a plinian erupt ion, remains one of the least understood processes in volcanology, alt hough assumptions about the timing and mechanisms of fragmentation are key parameters in all existing eruption models. Despite evidence to t he contrary, most models assume that fragmentation occurs at a critica l vesicularity (volume percent vesicles) of 75-83%. We propose instead that the degree to which magma is fragmented is determined by factors controlling bubble coalescence: magma viscosity, temperature, bubble size distribution, bubble shapes, and time. Bubble coalescence in vesi culating magmas creates permeability which serves to connect the dispe rsed gas phase. When sufficiently developed, permeability allows subse quent exsolved and expanded gas to escape, thus preserving a sufficien tly interconnected region of vesicular magma as a pumice clast, rather than fully fragmenting it to ash. For this reason pumice is likely to preserve information about (a) how permeability develops and (b) the critical permeability needed to insure clast preservation. We present measurements and calculations that constrain the conditions (vesicular ity, bubble size distribution, time, pressure difference, viscosity) n ecessary for adequate permeability to develop. We suggest that magma f ragments explosively to ash when and where, in a heterogeneously vesic ulating magma, these conditions are not met. Both the development of p ermeability by bubble wall thinning and rupture and the loss of gas th rough a permeable network of bubbles require time, consistent with the observation that degree of fragmentation (i.e., amount of ash) increa ses with increasing eruption rate.